Improve DefaultMetadata's TabletMap tablet invalidation logic (#388) #779

Job	Run time
Build (8)	2m 4s
Unit tests (8)	2m 11s
Setup ITs	3s
Full verify (8)	1m 51s
Cassandra ITs (RELEASE-3.X, 8)	20m 45s
Scylla ITs (ENTERPRISE-RELEASE, 8)	10m 49s
Cassandra ITs (RELEASE-4.X, 8)	23m 41s
Scylla ITs (ENTERPRISE-PRIOR-RELEASE, 8)	29m 31s
Scylla ITs (OSS-RELEASE, 8)	10m 40s
Scylla ITs (OSS-PRIOR-RELEASE, 8)	11m 7s
	1h 52m 42s

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